The present invention relates generally to electrode constructions for electrochemical systems, and particularly to flow-through porous electrodes for energy storage battery systems having circulating electrolytes.
Energy storage battery systems of the type referred to herein include one or more of the metal-halogen battery systems, such as a zinc-chloride battery system. These metal-halogen battery systems generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metal-halogen electrolyte. The electrolyte circulation subsystem operates to circulate the metal-halogen electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, where it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 Molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chlorine ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 Molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 Molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 5,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method Of using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
The present invention is directed to an improved flow-through porous electrode structure for electrochemical systems having circulating electrolytes, and which is also especially advantageous in zinc-chloride battery systems. Prior unipolar porous electrode structures for zinc-chloride battery systems have generally comprised two electrode members which were either separated by a non-conductive frame or profiled to form an internal cavity between the two members. One of these two approaches is discussed in detail in the commonly assigned Carr et al U.S. Pat. No. 4,241,150, entitled "Method For Control Of Edge Effects Of Oxidant Electrode", which is hereby incorporated by reference. A detailed discussion of the other of these approaches may be found in the "Development of the Zinc-Chloride Battery for Utility Applications" report, May, 1980, identified above. While these prior approaches have their respective advantages, one drawback is their relative vulnerability to bowing and breakage. With the internal cavity of these prior approaches, a substantial portion of the surface area of the electrodes members was unsupported with respect to transversely directed forces. Accordingly, the electrolyte flowing through the porous electrode members from the internal cavity would have a tendency to bow both electrode members outwardly. Although efforts have been to reinforce the two electrode members of these prior designs, these efforts have been limited to reinforcing only the center of the electrodes, such as by employing a pin to connect the two electrodes together at the center.
When graphite is used to construct the porous electrode members in zinc-chloride battery systems, these electrodes typically undergo a "activation" process to decrease the oxidation and reduction chlorine overvoltages of these electrodes. A detailed discussion of typical activation processes may be found in the following commonly assigned patents: Hart, U.S. Pat. No. 4,120,774 entitled "Reduction Of Electrode Overvoltage"; Laetham et al, U.S. Pat. No. 4,273,839 entitled "Activating Carbonaceous Electrodes". The specific teachings of these references are hereby incorporated by reference. Due to the nature of these activation processes, the porous graphite electrodes are placed in a substantially more stressful environment than encountered during normal battery operation. Depending on the intensity and length of these activation processes, the tendency of the prior porous electrodes to bow or break is exacerbated due to the severity of this environment. As will be appreciated by those skilled in the art, any deformation of the electrodes will alter the cell geometry, and thereby adversely affect the performance of the cell.
Accordingly, it is a principle object of the present invention to provide an improved flow-through porous electrode structure for electrochemical systems having circulating electrolytes.
It is a more specific object of the present invention to provide a flow-through porous electrode structure which has increased mechanical strength to resist any tendency towards bowing, breakage, or other deformation.
It is another object of the present invention to provide a flow-through porous electrode structure which has the ability to withstand a severe activation process to achieve enhanced voltaic performance of the electrode and/or decrease the time required for activation.
It is a further object of the present invention to provide a flow-through porous electrode structure which controls the flow of electrolyte through the electrode, yet also achieves a uniform distribution of electrolyte flow through the electrode.
It is an additional object of the present invention to provide a flow-through porous electrode structure for a zinc-chloride battery system which may readily be employed in a comb-type cell element providing the basic building block for constructing electrode stacks.
To achieve the foregoing object, the present invention provides a flow-through porous electrode structure generally comprising a porous electrode having two planar opposing faces, each of which provides a reaction surface for electron-transfer reactions of the same polarity. The porous electrode includes spaced passage means which extends at least in part through the electrode between the opposing faces for distributing the circulating electrolyte through the electrode. In a preferred form of the invention, the porous electrode comprises a single electrically conductive porous member which has the same outer dimensions as those employed for prior electrode structures of the same type. The passage means comprises a plurality of vertically spaced, generally horizontal disposed passageways extending from a first side of the porous member. These passageways have a generally circular cross-section and extend substantially across the width of the porous electrode member.